Answer:
The flat land was good for farming and so the landowners built very large farms called plantations. The crops that were grown were called cash crops
The correct answer is Indus.
The Yellow River was an important part of Chinese civilization.
The Tigris River was an important part of the Mesopotamia civilization, located in modern day Iraq.
The Amazon River is located in South America, and was an important part of several different civilizations.
This shows that the only correct answer can be Indus River.
